The invention relates to a filling level measuring device working on the principle of the transit time measurement of electromagnetic waves in the frequency range of below 3 GHz, in particular of below 2.5 GHz.
For the filling level measurement, measurement systems are used, which determine the distance between sensor and filling products on the basis of the measured transit time of electromagnetic waves from a sensorxe2x80x94in general also designated as filling level measuring devicexe2x80x94mounted in a receptacle cover, to the surface of the filling products and back. With knowledge of the receptacle height, the required filling level can then be calculated. Such filling level measurement devices, also known under the technical term filling level radar, are all based on the property of electromagnetic waves of propagating within a homogenous, nonconductive material at a constant speed and of being reflected at least in part at the boundary surface of different media. Each boundary surface of two media having different dielectric constants generates a radar echo upon impingement of a wave. The larger the difference between the two dielectric constants, the more the wave resistance of the wave propagation alters, and the greater is the echo to be observed.
For determining the required wave transit time, various radar principles are known. The two principally used methods are, for one, the pulse transit time method (pulse radar), and, for another, the frequency-modulated continuous wave method (FMCW radar). The pulse radar uses the pulse-shaped amplitude modulation of the wave to be radiated, and determines the direct time interval between transmission and reception of the pulses. The FMCW radar determines the transit time in an indirect way by emitting a frequency-modulated signal and the differentiation between the emitted and the received instantaneous frequency.
Apart from the different radar principles, different frequency ranges of the electromagnetic waves are also used, depending on the application. Thus, pulse radars exist, for example, having carrier frequencies comprised between 5 and 30 GHz, and there exist others working in the base band as so-called monopulse radars without carrier frequency.
Moreover, a series of methods and devices are known guiding the electromagnetic wave to the filling product surface and back. Thereby, the basic difference is made between a wave radiated into the space, and a wave guided through a line. With waves radiated into the space, a differentiation into so-called bistatic arrangements and so-called monostatic arrangements is possible.
Bistatic arrangements have two separate antennas, one of these serving for emitting and the other for receiving. In monostatic arrangements, a single antenna serves at the same time for sending and receiving.
Sensors with guided electromagnetic waves working on the reflection principle, normally are always configured monostatic. An advantage of a monostatic sensor realization results in saving a second antenna or waveguide terminal, and therewith in a simple, space-saving structure. A disadvantage arises by the circuit-technical necessity of connecting the transmitter circuit, as well as the receiver circuitxe2x80x94these are also designated as transmitting and receiving meansxe2x80x94with an antenna or a waveguide terminal connected upstream thereof. Thereby, the problem arises of mutually isolating the transmitter and receiver despite the common connection to the antenna and the waveguide terminal. Isolation means in this case the prevention of a signal change-over from the one circuit part to the other. Without this isolation, the transmitted signal would reach the receiver in the direct way, and would generate a comparatively large reception signal due to its relatively high amplitude.
Compared to the intended reception signals, which result from reflections on the measurement distance, and which are of a more or less high attenuation due to the transmission distance and the reflection at the measurement object, the transmitted signal, that reaches the receiver without isolation, causes a reception amplitude which is higher by a multiple. Despite the fact that this transmitted signal registered in the receiver arrives earlier from the measurement object than the echo to be evaluated, and that it is therewith not directly interfered with useful echoes, serious problems can arise due to the lacking isolation complicating or even preventing a distance measurement. Under these circumstances, the amplitude dynamics of the receiver must be adapted to a much higher range. In case the receiver cannot process the high amplitude of the transmitted signal, and even a limiting effect occurs, such an effect can remain effective for some time, until the time interval in which the useful echoes have to be expected. As a consequence thereof, the useful echoes would not be correctly receivable, and the measurement result would be questioned. In addition, it has to be expected that the impedance matching of the receiver is not ideal. A minor portion of each incoming echo signal is again reflected at the receiver input, and usually arrives at the receiver after a further reflection within the circuit, a second or third, etc., time. This phenomenon which can be characterized as ringing, plays no role in usual echo signals originating from reflections within the measurement distance. With a lacking isolation between transmitter and receiver, however, these signals of the transmitted signal, which have been reflected several times, are still in the order of useful echo amplitudes despite a corresponding attenuation. Due to their temporal position following the transmitted signal, they are interfered with reflections of measurement objects placed closed to the sensor. This leads to measurement errors in the proximity of the measurement device.
For interconnecting the sender, receiver and antenna or waveguide terminal, the following solutions are known. In the document DE 42 40 492, the use of a directional coupler or circulator is proposed. As to the practical circuit configuration of such a circuit part, no indications can be found therein. But it is described how a reception mixer can likewise be realized with directional couplers in such a way that the receptions signal and the local oscillator signal are mutually isolated. The directional couplers in this publication are realized as so-called hybriding couplers in microstrip technology. Similar directional coupler solutions can also be found in the U.S. Pat. No. 3,621,400.
The microstrip technology, however, is only applicable in a reasonable manner for signal frequencies in the microwave range (3-30 GHz). Usual radar sensors, however, use not only carrier frequencies, as has already been mentioned, e.g. in the range between 5 and 30 GHz, but there are also monopulse radars, which emit short pulses without using carrier frequencies. The frequency range of these transmitted signals typically is in the range of some Megahertz up to a few Gigahertz. Therewith, a directional coupler realization in microstrip technology is out of question for such monopulse radars.
In the document U.S. Pat. No. 5,517,198, a possibility is described, how in such a pulse radarxe2x80x94often also designated as TDR (time domain reflectometry)xe2x80x94a wide band isolation can be achieved between the transmitter and the receiver. The solution described therein uses a resistor bridge for dividing the transmitted signal into two identical portions. The one portion is directly fed into a branch of the receiver, the other one arrives in a second branch of the receiver, as well as in the measurement distance. Reflections from the measurement distance preponderantly arrive only in the second reception branch. By a differentiation between the two reception branches, the transmitted signal is eliminated from the receiver, while the reflections from the measurement distance remain uninfluenced. This solution is relatively extensive, since two reception branches are provided, and high amplitude attenuations of the useful echo are generated by the signal distribution over resistors.
In the document U.S. Pat. No. 5,609,059, another transmitting and receiving unit of the TDR filling level measuring device is described, by means of which undesired effects are reduced in the reception of a reflected electromagnetic signal. The function principle resides in a measurement of a difference of the transit time of an artificial reflection signal generated intentionally and the signal reflected at the filling product surface. The artificial reflection signal is generated by a resistor matched up to the impedance of the transmission cable and which is connected in parallel to the transmission cable. The signals reflected at the filling product surface are received and evaluated within a time window, which is pulsed by a control unit.
Another method for suppressing undesired interference phenomena in the signal response is described in the U.S. Pat. No. 5,365,178. Here, the signals reflected from the filling product surface are fed to a mixer and are compared with the frequency-modulated signals emitted by the transmitting unit, resulting in a frequency difference depending on the filling level. In this differential signal, undesired interfering frequencies are contained arising by the reflection of the signals at the inner receptacle side or in the transmitting and receiving unit itself. These undesired signals are without the frequency range of the desired signals and are eliminated by means of a band pass filter.
The technical problem on which the invention is based resides in providing a monostatically structured filling level measuring device, in particular a base band radar or TDR filling level measuring device, allowing a simple, cost-efficient combination of transmitter, receiver and antenna or waveguide terminal, guaranteeing a high degree of isolation between transmitter and receiver. This combination of transmitter, receiver and antenna or waveguide terminal should moreover be of such a wide band configuration that it transmits, such as it is for example necessary with the monopulse radar, a frequency band of between some Megahertz and a few Gigahertz. The amplitude attenuation of the transmitted/received signals thereby should be as low as possible.
The invention is further based on the technical problem of proposing a simple and cost-efficient method for transmitting and receiving electromagnetic waves having a frequency of below 3 GHz, via a single antenna means.
These technical problems are solved by means of a filling level measuring device having the features of claim 1, and by a method having the features of claim 20, respectively.
An inventive filling level measuring device, which is based on the principle of transit time measurement, comprises an electronic unit with a transmitting and receiving means for electromagnetic waves having a frequency of below 3 GHz, a coupling means connected to said electronic unit which guides the transmitted signals onto the measurement distance and guides reflected signals back to the electronic unit, and a high-frequency transformer to which the transmitting and receiving means and the coupling means are connected.
An inventive method is characterized in that transmitted signals of a transmitting means are guided into the antenna means via a high-frequency transformer, and the reflected signals received by the antenna means, are guided to a receiving means via the same high-frequency transformer.
Through the first-time use of a high-frequency transformer functioning in a wide band manner in the frequency range of up to about 3 GHz, in particular of up to about 2.5 GHz, as a directional coupler by means of a corresponding geometry of the turns and wiring, and allowing a defined isolation between determined circuit components for transmission and reception connected thereto, an extremely cost-efficient, as well as effective solution is provided. It has finally to be noted, that here, under an antenna means, one or more waveguides have, for example, to be understood such as they are known in prior art for guiding the electromagnetic waves and the signals reflected from the filling products.
An important advantage of the inventive solution resides in that a good matching of the impedances of the individual components, in particular of the input impedance for the signals reflected from the measurement distance is possible. With a mismatching, the signals would be reflected several times, and additional interfering echoes would arise.
The determination of the ratios of the turns enables any desired impedance transformations, so that the impedance matching between individual circuit components can be optimized. So-called double hole transformers in SMD technology have turned out to be particularly suited. By a corresponding ferrite material and a corresponding winding of the double hole core, these transformers transform frequencies of up to 3 GHz, in particular of up to 2.5 GHz.
For these transformers, there exist two basically different winding arrangements, which can be used in different ways for the solution of the above-mentioned technical problem. The one group is here categorized under the term differential transformers, the other group of transformers is designated as transformers-directional couplers according to their purpose of use. Both types are known, in principle. Whereas differential transformers usually are used in frequency mixers and as baluns, transformers-directional couplers are mostly used in the distribution of satellite TV reception signals. In this application, a certain portion is decoupled via the directional coupler from a transmission cable for the connection of a receiver, whereas the preponderant signal portion is further guided on the main line.
In certain cases, it has turned out to be advantageous when the isolation between the transmitting and the receiving means is not completely ideal, rather allowing itself to be definably reduced. This is of importance there, where a sensor uses as a distance reference label that part of the transmitted signal reaching the receiver directly, due to an isolation which is not entirely perfect. A sensor working in this way is, for example, described in DE 42 40 491.
Preferably, the high-frequency transformer is a differential transformer featuring windings magnetically coupled with each other, two of which being connected to each other. This simple construction is cost-efficient and very purposeful for the adjustment of the respective impedances. In that all of the three windings feature the same number of turns, the adjustment of the impedances of the components connected to the differential transformer can be realized at all gates until the ideal impedance matching is reached. Thereby, the received echo signal is not subjected to additional reflections, and can therewith not cause any distortion of the measurement result. With this construction, it is also possible without any problems to carry out an impedance matching, so that a complete isolation is not given between the gate to which the transmitted signal is applied, and the gate from which the reception signal is emitted. In an advantageous manner, this signal portion immediately further transmitted from the transmitter to the receiver can, as has been outlined before, be used as a distance reference label.
Another preferred embodiment resides in that the differential transformer comprises two windings featuring the same number of turns, the third winding, however, comprising a turn number different therefrom, in particular comprising twice as many turns as one of the two first mentioned windings. Thereby, the impedance of the receiver means necessary for polydirectional matching is four times as high as with the turn ratios of the embodiment discussed before. The higher receiver impedance is often advantageous, since receiver means in many cases can be realized more easily at higher resistance than at lower resistance. With the receiver impedance being given, an optimum impedance matching can be allowed for by a matching of the turn ratios of the differential transformer. Depending on the given facts, however, it is also possible to provide other winding ratios.
It general, it can be stated that through the use of a high-frequency transformer, a low-resistance discharge of a static charge present on the waveguide is possible, which occurs in particular, e.g. when the filling product is granular plastic material. Through friction of the filling product at the waveguide during the filling or emptying process, same can become statically charged. The very high voltages arising thereby, can destroy the electronic circuitry connected to the waveguide when they are not connected to ground via the receptacle mass. This destruction is prevented through the simple low-resistance connection between the waveguide or the process coupling and the receptacle mass.
With the use of a transformer-directional coupler as the high-frequency transformer, in contrast to the differential transformer, ensues not a uniform but always a non-uniform signal division. Thereby ensues a greater signal attenuation than with the differential transformer, but it is advantageous that in the event of reception, mainly the receiver impedance has to be modified for the reflection-free signal matching. The appropriate impedance matching of the transmitter probably is more difficult.
Finally, it has to be emphasized that ring-shaped cores wound with three filaments can also be used in the sense of the present invention. Here, the wide band nature results from the specific kind of winding enabling a capacitive line coupling between the turn wires at high frequencies.